Arsenicals have been used in medicine for centuries however their re-emergence in the treatment of cancer has only occurred in the last 15 years. This has been based on the remarkable activity of arsenic trioxide in the treatment of Acute Promyelocytic Leukemia (APL). In APL, arsenic appears to target the oncogenic lesion associated with this disease, however it is clear that arsenicals have activity in other tumor types. Multiple myeloma, a neoplasia of the antibody secreting cells of the bone marrow is one such disease. Several trials have demonstrated that arsenic trioxide has activity alone and in combination with other chemotherapeutic agents therefore understanding the mechanism of action of arsenicals in this disease is warranted. In previous studies and preliminary data presented within this application we have demonstrated that arsenic trioxide induces apoptosis in myeloma cell lines and patient samples and that depletion of glutathione can enhance this effect. This has resulted in a phase I/II clinical trial to test the safety and efficacy of the combination of arsenic trioxide and ascorbic acid in refractory/relapsed myeloma. We now demonstrate that gene expression profiling of the response to arsenic demonstrates both a protective antioxidant response via the activation of Nrf2 and a pro-apoptotic response via activation of the BH3 only proteins Noxa and Bmf. The goals of the first Specific Aim of this application are to determine the mechanism of activation of Bcl-2 family members by arsenic. In the second Specific Aim we will extend our studies to a novel organic arsenical, SGLU (ZIO-101) that can also kill myeloma cell lines and is also currently in clinical trials including for myeloma. We have determined by gene expression profiling that SGLU does not activate the anti-oxidant response but does activate Noxa. Therefore we will determine the mechanisms of uptake, metabolism and action of this novel arsenical. In the final Specific Aim we will characterize an arsenic-resistant variant of one of the myeloma cell lines that we have been using to learn more about both arsenic mechanism of action as well as potential resistance mechanisms. Together these studies will provide novel insights into arsenical mechanism of action and for rationale designed combination therapies. PUBLIC HEALTH RELEVANCE: Arsenic trioxide is FDA approved for the treatment of the Acute Promyelocytic Leukemia (APL) and is being tested in several other diseases including the bone marrow cancer Multiple Myeloma. While the mechanism of action in APL is well defined it remains unclear why other cancers are sensitive to this agent. We have determined that two pathways are activated by arsenic trioxide in myeloma cells, one that could be protective to the cell and another that would kill the cell and will now define the roles of each of these pathways as well as determine the mechanism of action of a new arsenic-based therapeutic agent that is being tested in clinical trials.